1. Field of the Invention
The present invention relates to a hydrogen storage alloy electrode which is capable of reversibly absorbing and desorbing hydrogen in electrochemical process.
2. Description of Related Art
There have been lead-acid batteries and alkaline batteries widely used as storage batteries cells for various power supplies. Among them, the alkaline storage batteries have been expected to have a higher reliability and made miniature with a lighter weight. For the reason, the miniature type of alkaline storage batteries has been employed in various portable apparatuses while the large type has been employed in industrial applications.
In the alkaline storage batteries, positive electrodes have been made of nickel in most cases though there have been employed partly air and silver oxide as positive electrodes. The storage batteries have been improved in their performance as the electrodes have been changed from a pocket type to a sintered one, and have found ever broadening areas of application since the storage batteries were allowed to be sealed.
On the other hand, most negative electrodes are made of cadmium at present, though zinc, iron, hydrogen and the like besides cadmium have been considered for negative electrodes. In order to achieve a higher energy density, however, an interest has been directed to nickel-hydrogen storage batteries using metal hydrides, i.e., hydrogen storage alloy electrodes, and many proposals have been made for the production thereof.
The hydrogen storage alloy electrodes to be used as negative electrodes in the alkaline storage batteries, where the electrodes are made of hydrogen storage alloy capable of reversibly absorbing and desorbing hydrogen, have a higher theoretical capacity density than that of cadmium electrodes, and do not cause such a deformation, nor a formation of dendrites as may occur in the case of zinc electrodes. Therefore, they have a prolonged useful life and do not harm the environment so that they have been expected as negative electrodes for use in the alkaline storage batteries having a higher energy density.
As alloys to be used for the hydrogen storage alloy electrodes as above, multi-component alloys such as Ti--Ni and La(or Mm)--Ni systems have been generally well known. The multi-component alloy of the Ti--Ni system may be classified as an AB type (A is an element having a high affinity for hydrogen such as La, Zr, Ti and the like; B is a transition element such as Ni, Mn, Cr and the like), and is characterized by exhibiting a relatively high discharge capacity at an initial stage of charging and discharging cycles, but produces a problem that it is difficult to sustain the capacity for a long period of time while repeating the charging and discharging cycles. Another multi-component alloy, La(or Mm)--Ni system of an AB.sub.5 type has been extensively studied recently as electrode material, and regarded as more promising alloy material so far. However, this alloy system also produces problems of a relatively small discharge capacity, insufficient performance in the useful life as cell electrode, high material cost and the like. Therefore, there has been a need for novel hydrogen storage alloy material allowing a higher capacity and longer useful life.
In contrast, Laves phase alloy of an AB.sub.2 type has a higher hydrogen absorbing ability, and has been considered promising for electrodes having a high capacity and long life. In this alloy system, there have been already proposed, for example, Zr.alpha.V.beta.Ni.gamma.M.delta. alloy [Japanese Patent KOKAI (Laid-open) No. Sho 64-60961], ZrMo.alpha.Ni.beta. alloy [Japanese Patent KOKAI (Laid-open) No. Sho 64-48370], and AxByNiz alloy [Japanese Patent KOKAI (Laid-open) No. Hei 1-102855].
When the Laves phase alloy of the AB.sub.2 type is used for electrodes, a higher discharge capacity and a possible longer life can be realized as compared with those obtainable using the multi-component alloys of the Ti--Ni system or the La (or Mm)--Ni system. However, a further enhancement of those performances are still being sought for the storage battery application.